Lithium lanthanum tantalate (LLTO) ceramics exhibit broad technological significance as lithium-ion conductors, with particular relevance for applications such as lithium-ion battery systems. More recently, LLTO ceramics have been identified as useful materials for nuclear waste separation and fuel reprocessing due to a difference in Li and Cs diffusivity. See U.S. Provisional application Ser. No. 14/660,696, filed Mar. 17, 2015, which is incorporated herein by reference. For optimal ion conductivity and robust structural and chemical stability, however, lithium-ion conducting ceramics must be highly dense with no interconnected porosity. High density ceramics can also exhibit higher total conductivity due to their apparent lack of volumetric pore dilution.
Pressureless sintering is an important processing requirement for production of low cost ion conductors. Pressure assisted densification (e.g., hot pressing), although often an effective means of achieving high density ceramics, is generally considered a batch-type process that requires significant capital investment and recurring tooling costs. Moreover, it can be difficult or impossible to create contoured ceramic geometries (e.g., tubes) using hot pressing. As a result, processing methods that enable pressureless sintering are often favored over hot isostatic pressing or hot pressing.
LLTO is an alkali containing ceramic, and alkali ceramics are particularly difficult to sinter to full density for several reasons. The Li2O—La2O3—Ta2O5 phase diagram is very complex, making LLTO processing difficult. See K. Hayashi et al., Mat. Res. Bull. 21(3), (1986). Further, alkali oxides (R2O, e.g., Li2O) exhibit significant vapor pressures at common sintering temperatures, leading to significant reagent loss to volatility. See R. H. Lamoreaux and D. L. Hildenbrand, J. Phys. Chem. Ref. Data 13(1), (1984). Common sintering temperatures for alkali-based ceramics are frequently above 1000° C. which exacerbates the effects of R2O volatility. Alkali-based ceramic particles are also prone to coarsening due in part to R2O volatility as well as the propensity to form liquid phases during calcination. If the alkali is allowed to escape from the atmosphere surrounding the ceramic, a typical microstructure will exhibit a porous outer rim and semi-dense interior. Therefore, the composition of the sintered ceramic depends on location within the specimen and will be alkali deficient near the outermost regions of the sintered ceramic. Finally, Li2O(g) is reactive with most oxide kiln hardware in which many sintering studies are conducted.